/Engine/Source/Runtime/SlateCore/Public/Widgets/SWindow.h
Sets a widget to use as a full window overlay, or clears an existing widget if set. When set, this widget will be drawn on top of all other window content.
void SetFullWindowOverlayContent ( [TSharedPtr](API\Runtime\Core\Templates\TSharedPtr)< [SWidget](API\Runtime\SlateCore\Widgets\SWidget) > InContent )